Radioimmunoassay of free thyroxine in serum: comparison with clinical findings and results of conventional thyroid-function tests.

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Abstract
We have evaluated a radioimmunoassay for free thyroxine (FT4) involving antibody-coated tubes (GammaCoat 125I Free T4RIA; Clinical Assays, Div. of Travenol Labs, Inc.). The coefficient of correlation between FT4 and the FT4 index was 0.98 for all patients with various thyroid disorders, 0.77 for hospitalized patients with miscellaneous diseases, and 0.74 for healthy individuals. FT4 values also agreed well with triiodothyronine or thyrotropin concentrations in these patients and were consistent with each patient's clinical status. Patients with severe nonthyroidal illnesses and abnormal thyroid-function tests, despite clinical euthyroidism, had normal FT4 values. In this group, the mean FT4 was almost identical to that in the controls, although mean thyroxine, triiodothyronine, and FT4 index differed significantly and the correlation between FT4 and FT4 index was poorer (r = 0.66). For most patients, the diagnostic value of FT4 measurements is comparable to that of the FT4 index, and it may be superior in patients with severe nonthyroidal illnesses. The test costs less and saves time as compared to the FT4 index computation, and it can be used routinely with thyrotropin assay for the diagnosis of hypothyroidism or with triiodothyronine assay for the diagnosis of hyperthyroidism.